As an accumulator device having a high power and a high energy density, attention has been recently paid to a lithium ion capacitor. However, when the conventional lithium ion capacitor has designed to have a higher energy density, the internal resistance becomes higher. Thus, a problem of trade off relationship is caused.
Patent Literature 1 discloses a lithium ion capacitor that has a lower resistance by appropriately adjusting the ratio of the weight of a positive electrode active material and that of a negative electrode active material.
However, in the lithium ion capacitor described in Patent Literature 1, the internal resistance is not sufficiently reduced. Therefore, the lithium ion capacitor satisfies neither a high power property nor a high energy density.